^ On this I must disagree. There are a number of arguments used to support the claim that slavery was only a minor issue, but they really don't stand the test of closer examination. I am going to address those of them I can think of spontaneously.
It is true that when the civil war started slavery was not proclaimed as a casus belli. Abraham Lincoln had announced that he was not going to do anything against slavery in the states where it already existed, but he did intend to prevent the spread of slavery in the new territories. This most likely would have led to an end of slavery in medium term with the increasing number of Senators from free states over those from slave states. For a very long time the number of free and slave states had been kept in ballance so the senators from the south could block any attempt to abolish slavery (due to the lower population numbers the southern representatives in the house were outnumbered in spite of 3/5 of the slaves being counted when determining the number of representatives for a state). In the 1850s the ballance was no longer maintained 15 states permitted slavery while in 19 it was prohibited after the last six states to join the Union before the outbreak of the Civil War had been free states. Especially in case of "Bleeding Kansas" there had been violent confrontation between abolitionists and supporters of slavery.
What is interesting to see is that some neo conservatives today pretend that the gradual end of slavery as probably envisioned by Lincoln had been the confederate's intention as well. And even in the 1915 movie "Birth of a nation" which contributed a lot to the rebirth of the KKK Lincoln is claimed as "the south's best friend":
Anyway, it is true that only about a fourth of the souther population did hold slaves and most of them held but a single one with the posession of hundreds of slaves in the hands of a relatively small elite of plantation owners making up for a very large part of the total slave population. Nonetheless it is a sad fact that even to the poorest of the poor among the white population who could not afford slaves it was an important psychological factor not to be at the absolute deepest spot in the pecking order which was reserved for the slaves.
It is also true that it took more than a year after the outbreak of the Civil War for the abolition of slavery to be proclaimed an official goal of the war in the north. Lincoln had prepared a preliminary proclamation of emancipation in the summer of 1862 but agreed with his advisors who (in this time of confederate victories) recommended to wait for a victory to issue the proclamation to avoid the impression of the proclamation as a last desperate measure. In the aftermath of the battle of Antietam (bloodiest day in US history and a missed chance for both parties of the war to turn the war in their favor, but still a strategic success for the north by the necessity for Robert E. Lee to withdraw from Maryland after the battle) Lincoln issued the proclamation. This first proclamation didn't free a single slave at the time it was issued (in september 1862), but it announced the emancipation of all those slaves held in parts still in rebellion against the US by January 1st 1863. Nothing was said about the four slave states that had remained in the Union or the slaves in the parts no longer in rebellion because of their conquest by the north (e.g. the town of New Orleans), but it was clear that after that proclamation the issue of slavery had been lifted officially to the status of a casus belli. Had it been officially proclaimed as the aim of the war when the war broke out, it is quite likely that many people in the north would not have been willing to fight for it, for sure enough racism was by no means limited to the south. A major advantage of the issuing of the proclamation of emancipation was that it made a diplomatic recognition of the south by European nations (England and France) practically impossible and without their support a military victory of the South was practically impossible for the south with the only hope remaining being the draining of the northern will to fight this war (a will which I think has often been underestimated).
It is often said that economic reasons were the main reason for the Civil War. One may say so, but the economy of the south was to a very large degree based on slavery, so its abolishment would indeed pose a very important economic reason. As for economic reasons not directly linked to slavery however the situation looks quite differently. Take the
nullification crisis of 1832 for example. Back then South Carolina threatened to seccede from the union over the issue of federal tariffs if the government would try to enforce the tariffs. South Carolina did not get the backing from the other southern states which South Carolina had apparently expected and it became rather distinct that it was not an issue the majority of people in the south was willing to fight a war about.
A confederate soldier and POW when asked by a northerner why he was fighting for slavery when he was obviously not wealthy enough to even own a slave is reported to have responded: "I fight because you are down here [in the south]." This indeed likely echoed the sentiments of many a confederate soldier who felt that they were defending their homeland against invaders from the north. However, the issue that had ultimately brought the northerners down there had been the southern seizing of and attacks on federal property (Fort Sumter) after a declaration of secession issued mostly because of the issue of slavery.
A favorite term among neo-confederates is "State rights" and the claim that the right of states for self determination was what the south was fighting for. This sounds a lot more politically correct. However, in the years preceeding the civil war it was not so much the north violating the perceived right for slavery in the south as it was the south not accepting the right of the northern states not to accept slavery there. The Dred Scott decision of 1857 is the best example for this. The slave Dred Scott had been living for more than two years as a slave to Dr. John Emerson in the free(!) state of Illinois to which Emerson had taken him after purchasing him in 1833. Next Scott was taken to the Wisconsin Territory (a part which would later belong to Minnessota). After returning to Slave state Missouri Emerson died in 1842. Now, after his masters death and after having spend almost a decade in slavery in parts of the US where slavery was prohibited Scott demanded his freedom. Ignoring the state right NOT to permit slavery within the own borders the court decision determined that no time spend in free territory could lift the slave status of a slave. In the fugitive slave laws it was even made a crime to support runaway slaves or try to prevent their recapture in free states and their forcible return to the slave states. The question must be permitted whose state rights were actually violated here?
